This invention relates to a method of treating a substrate, in particular, although not exclusively, to shallow trench isolation (STI) of semiconductor devices on a substrate, and also to intermetal dielectrics (IMD).
According to WO 94/01885, it is known to achieve planarisation of a depositing layer by depositing a liquid short-chain polymer formed from a silicon containing gas or vapour on a semiconductor wafer.
In a shallow trench isolation (STI) process, semiconductor devices on a wafer are packed tightly together by means of trenches filled with insulating material. In such cases, trench widths tend to vary between 0.13 to 0.35 microns, and trench depths are typically between 0.3 and 0.5 microns. If the depositing material of WO 94/01885 is used in the shallow trench isolation process, the integrity of the dielectric can be tested by cleaving a sample perpendicularly through a trench, dipping the cleaved edge in a silicon dioxide etchant such as 10:1 buffered hydrofluoric acid (BHF) and analysing the cleaved edge in a scanning electron microscope (SEM). However, it has been found that a problem occurs within the trench after de-lineation (etching). In this respect, it has been found that the material filling the narrow gaps may completely etch away from the walls of the trench and material filling wider gaps even shows severe etching from the bottom corners of the trench. Thus, such a shallow trench isolation process has been found to be unsatisfactory. It has been found that the material, when deposited in an STI process, tends to have an inconveniently fast wet etch rate at the top of and/or within the trench. This means that the trench plug does not stand proud of the trench upper surface, as required for effective shallow trench isolation.
In IMD layers there is a requirement to sometimes fill narrow gaps between metal lines as a result of very small pitch/close spacing.
There is therefore a need to provide an improved process.
According to a first aspect of the present invention, there is provided a method of treating a substrate having a silicon-containing polymer deposited in a recess on its surface, which method comprises the steps of:
(a) heating the substrate; and
(b) subjecting the surface of the heated substrate to gas or vapour activated by a plasma or other electromagnetic radiation other than heat alone.
The silicon-containing polymer layer may be deposited in any manner known in the art. Thus, the method is applicable not only to a method of depositing a layer as described in WO 94/01885, but is also applicable to spin coating processes, eg. xe2x80x9cspin on glassesxe2x80x9d (SOG). With SOG and other liquid precursor methodologies for depositing doped and undoped silicon oxide materials, wet chemicals are deposited on to a spinning wafer which is then further processed by including the heat treatment to form a stable polymeric oxide. However, in a preferred embodiment, the silicon-containing polymer is deposited on the surface of the substrate by positioning the substrate in a chamber, introducing into the chamber a silicon-containing gas or vapour and a compound, containing peroxide bonding, in vapour form, and reacting the silicon-containing gas or vapour with the compound to form the polymer. In such a case, the silicon-containing gas may be inorganic or organic, and is preferably a silane (for example silane itself) or a higher silane. The compound containing peroxide bonding is preferably hydrogen peroxide.
It has been found that preferred results are obtained when the gas or vapour activation is in the form of a plasma. The plasma treatment is more effective after a period of heat treatment and, without restricting the applicant hereby, it may possibly be due to water present in an unheated layer absorbing or interfering with some radiation characteristic of the plasma. Whilst it has been found that an oxygen plasma may produce some improvement, a CF4 plasma is particularly preferred.
The heating of the substrate, which takes place before subjecting the substrate to the activated gas or vapour, may be provided by any suitable means, but in a preferred embodiment is provided by placing the substrate on a heated platen. The preferred temperature for the heating step is 300xc2x0 C. to 500xc2x0 C. In a particularly advantageous embodiment, the substrate is positioned on a platen which is connected to an RF power source. This is in contrast to systems in which the RF source is connected to a xe2x80x9cshower headxe2x80x9d through which reactants are passed into the chamber and the platen is earthed or electrically floating, or where the plasma is remote from the substrate. In a preferred embodiment of the present invention, the RF power source is a low frequency source and is preferably less than 1 MHz. The low frequency tends to create higher peak-to-peak voltages and a qualitatively different subatomic bombardment than for the same power when applied at a higher frequency (typically 13.56 MHz). As mentioned above, it has been observed that the process is more effective if the substrate is on the RF driven electrode, and therefore subject to a greater level of subatomic bombardment, particularly of ions, than if it were on a floating or grounded electrode.
It is generally the case that the higher the temperature and the higher the peak-to-peak voltages achieved on the substrate, the more effective the treatment becomes.
However, this is not to say that a process where a gas or vapour is ionised by a plasma remote from the substrate or an electrodeless plasma, eg. inductively coupled, or a plasma where the substrate is on a grounded or floating electrode will not be effective.
The method may further comprise the step of depositing or forming a capping layer on the surface of the deposited layer. This capping layer, if used, is deposited after subjecting the substrate to the activated gas or vapour and, in a preferred embodiment, may be formed of SiO2.
In addition, the method may further comprise heating the polymer layer after capping, for example in a furnace.
The substrate has one or more recesses (or trenches) in its surface which are filled with the silicon containing polymer. The treatment described preferably densifies the polymer in the recess. Preferably, an underlayer of thermal oxide, for example silicon dioxide, is provided on the substrate under the deposited polymer. In addition, a mask, for example formed of silicon nitride, can be formed on the thermal oxide layer. The densification may provide a reduction in the wet etch rate of the polymer to one more similar to the thermal oxide underlayer. This provides improvement in the processing of shallow trench isolation of active zones, eg. gates, in the semiconductor device.
Whilst the applicant is not to be restricted hereby, the densification process is believed to be at least partially chemical or radiative in nature because it has been found that argon has no effect, oxygen has some and CF4 has a strong effect. Thus, the physical characteristics of the polymer layer appeared to be modified.
The method is particularly applicable to shallow trench isolation (STI) processes. Thus, preferably the recess width is between 0.13 and 0.35 microns and the recess depth is between 0.3 and 0.5 microns. Preferably, the silicon containing polymer extends from the recess above the upper surface of a semiconductor wafer. This is because, in the STI process, for separation of active silicon or the like in a wafer to be as complete as possible the isolation needs to extend above the surface of the active areas.
The method may also be usefully applied to intermetal dielectric (IMD) layers, in particular where the intermetal gaps are between 0.13 and 0.35 microns.
It is preferable to carry out the treatment with activated gas or vapour of the wafer in a separate chamber from the step of depositing the polymer. It has been found that, in a preferred embodiment, the treatment with activated gas or vapour can be carried out in the same chamber as the previous heat treatment.
In a preferred embodiment, the wafer is heated for 60 seconds at a platen temperature of 350xc2x0 C. The pressure in the chamber is preferably about 250 mTorr and the flow rate of the CF4, is about 200 SCCM (0.33 Pam3/S). In this preferred embodiment, a 380 kHz power source applies about 500 Watts to the platen, when used, for about 60 seconds. In general, temperature and pressure and power ranges may be limited by the preferable integration of this treatment into a process chamber already provided for the capping of the polymer.
Typical ranges might be:
The gas flow will depend on the pumping speed of the process chamber. It will be appreciated by those skilled in the art that a purpose-built chamber for this process may advantageously apply higher temperatures (for example up to 1200xc2x0 C.) and higher powers.
Thus, in accordance with a second aspect of the present invention, there is provided a method of isolating active zones in a semiconductor layer on a substrate having a layer of thermal oxide on its surface, comprising depositing a silicon-containing polymer in one or more recesses separating the active zones, heating the substrate, and subjecting the silicon-containing polymer to a gas or vapour activated by a plasma or other electromagnetic radiation other than heat alone such that the wet etch rate of the silicon-containing polymer is reduced to be similar to that of the thermal oxide.
This shallow trench isolation process may have the appropriate preferred features as mentioned above. For example, the method has been found to be particularly advantageous when the width of the one or more recesses is between 0.13 and 0.35 microns and the depth is between 0.3 and 0.5 microns. In a subsequent treatment step, the thermal oxide layer is preferably etched, for example using buffered hydrofluoric acid (BHF). The xe2x80x9csimilarxe2x80x9d wet etch rate referred to above is sufficiently similar such that the silicon containing polymer is capable of standing above the level of the surface of the substrate after the etching. Typically, the activated gas or vapour treatment reduces the wet etch rate of the silicon containing polymer, from 2 to 2.5 times faster than the etch rate of the silicon dioxide, to 1.2 times faster.
In accordance with a third aspect of the present invention, there is provided an apparatus for carrying out the methods referred to above.
Thus the apparatus may comprise means for heating the substrate and means for subjecting the heated substrate to gas or vapour activated by a plasma or other electromagnetic radiation other than heat alone.
The apparatus may comprise a chamber having a platen for supporting the substrate, and means for introducing into the chamber a silicon-containing gas or vapour and a compound containing peroxide bonding.
Although the invention has been defined above it is to be understood that it includes any inventive combination of the features set out above or in the following description.